1. Field of the Invention
The present invention relates to spectral devices, and more particularly to double grating monochromators. The invention can be utilized in the laboratory for making a variety of spectroscopic investigations requiring a high resolution or an increased aperture ratio, with a low-level scattered radiation.
2. Description of the Prior Art
One known double grating monochromator (cf. USSR Inventor's Certificate No. 600401, published in 1978) comprises fixed slits, viz. an entrance, an intermediate, and an exit slit, a pair of diffraction gratings disposed on one rotatable mount connected to a scanning device, and a set of mirrors to change the direction of the light flux passing through the monochromator.
In this monochromator, the slits after each of the diffraction gratings are positioned so as to extract radiation of the first spectral order. It imposes limitations on the resolution of the monochromator or on its aperture ratio for a given resolution. On the other hand, the extraction of higher-order radiation in said monochromator results in a narrower free spectral region which in turn restricts the overall spectral region of the monochromator. As is well known, the free spectral region .DELTA..lambda. is given by EQU .DELTA..lambda.=.lambda..sub.1 /K (1)
where
.lambda..sub.1 is lower bound of the spectral region, and
K is spectral order of radiation.
The above expression shows, that the higher the order of the radiation spectrum, the narrower is the free spectral region.
In order to overcome this drawback, it is theoretically possible that filters be employed to cut off radiation of unwanted spectral orders, but such filters find limited practical application, particularly in vacuum ultraviolet radiation region.
Also the aforementioned monochromator has a relatively low aperture ratio due to a great number of reflecting surfaces present in the structure. It is particularly critical in the vacuum UV radiation region, wherein the reflection ratio is not in excess of 0.5 (in the visible region, in fact, the reflection ratio is as high as 0.85).
Also known in the prior art is a double grating monochromator (cf. "Vestnik leningradskogo universiteta", Leningrad, 1973, No. 10, pp. 57-58, FIG. 1) comprising an entrance slit, a first diffraction grating, an intermediate slit, a second diffraction grating, and an exit slit all arranged in series along the propagation path of radiation and optically interconnected. As the radiation spectrum is scanned, the diffraction gratings are rotated by a scanning device mechanically connected with one of the diffraction gratings. Synchronous rotation of the other diffraction grating is accomplished by a lever mechanism linked with the mechanical transmission.
Both the halves of this monochromator also extract radiation of the first spectral order and exhibit identical parameters of the optical system, causing both gratings to be rotated through the same angle as the spectrum is scanned.
Compared to the monochromator as disclosed in the USSR Inventor's Certificate mentioned hereinabove, this monochromator provides a better aperture ratio owing to the fact that the number of reflecting surfaces is here minimized as a result of absence of the mirror array. Since this monochromator, however, also deals with the first spectral order radiation alone, it suffers from all the above disadvantages, namely: low resolution or low aperture ratio for a given resolution. The spectral region is further limited by the grating efficiency being reduced, as the angles of radiation incident on the gratings are removed, in the process of scanning, from a value corresponding to the grating positions that provide for the maximum power concentration of the outgoing radiation within the first spectral order ("blaze" effect). Moreover, when a lever mechanism is used in the scanning device, eventual plays as well as temperature and vibration effects prevent a sufficiently timed rotation of the diffraction gratings, resulting in a reduced accuracy of the monochromator unit.